[lib/LTO] Factor out logic for running passes.

This is in preparation for adding an option
to run a custom pipeline with the new PM. It's
currently used in lld.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280258 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed